Presentation on theme: "Gases Notes. 12.1 A. Physical Properties: 1.Gases have mass. The density is much smaller than solids or liquids, but they have mass. (A full balloon weighs."— Presentation transcript:
12.1 A. Physical Properties: 1.Gases have mass. The density is much smaller than solids or liquids, but they have mass. (A full balloon weighs more than an empty one.) 2. Gases can be compressed. It is very easy to reduce the volume of a gas. Example: air in a bicycle tire
3. Unlike liquids, gases completely fill their containers.
4. Gases can move through each other rapidly. Example: the diffusion of food smells and perfume.
5. Gases exert pressure.
6. The pressure of a gas depends upon temperature. high temp. = high pressure, low temp. = low pressure
B. Kinetic-Molecular Theory: 1. Gases are small particles that have mass. These particles are usually molecules, except for the Noble Gases.
2. The particles in gases are separated by relatively large distances.
3. The particles in gases are in constant rapid motion (random). 4. Gases exert pressure because their particles frequently collide with the walls of their container and each other.
5. Collisions of gas particles are perfectly elastic. Inelastic Collision Elastic Collision
Gas particles do not slow down when hitting each other or the walls of their container… How do we know?
6. Temperature of a gas is simply a measure of the average kinetic energy of the gas particles. High temp. = high KE, Low temp. = low KE
7. Gas particles exert no force on one another. Attractive forces are so weak between particles they are assumed to be zero.
C. Measuring Gases: 1.The following 4 variables will be used to do gas calculations: n - amount of a gas, it is measured in moles V - volume of a gas, it is measured in L (dm 3 ) or mL (cm 3 ) “STP” = Standard Temperature & Pressure: T- Standard Temperature: 0 o C = 273 K Taken in o C converted to Kelvin (K) T(K) = °C Ex #1) 22°C = 295 K 100.°C = 373 K -27.3°C = K -273°C = 0 K
P - Standard Pressure: can be measured using the following units: (sea level pressure) 1 atm (atmospheres) 760 mm Hg 101,325 Pa (Pascals)760 torr kPa (kilopascals)14.7 lb/in 2 (psi) Ex #2) Convert atm to kPa: Ex #3) Convert 98,500 Pa to mm Hg:
2. Atmospheric pressure is the pressure exerted by the air in the atmosphere. This pressure varies with altitude and water vapor content.
3. Atmospheric pressure is measured with a barometer. This is a glass tube sealed at one end and filled with Hg. Why toxic mercury and not water?
Atmospheric Pressure Affects Weather: Changes with Altitude:
Boiling Point Changes w/Altitude & Pressure
12.2 The Gas Laws A. Boyle’s Law: Pressure - Volume Relationship. The pressure & volume of a sample of gas at constant temperature are inversely proportional to each other. Indirect P 1 V 1 = P 2 V 2
Ex #4) A gas has a volume of 300. mL under a pressure of 740. mm of mercury. If the temperature remains constant, calculate the volume when under a pressure of 750. mm Hg. P 1 V 1 = P 2 V 2
B. Charles’ Law: Temperature - Volume Relationship. At constant pressure the volume of a fixed amount of gas is directly proportional to its absolute temperature. Direct *Temperatures must be in Kelvin! K = °C + 273
Ballon in cool and cold water:
Ex #5) What is the Celsius temperature of mL of methane, if it occupies a volume of L at 50.0°C? mL
C. Avogadro’s Law: Amount - Volume Relationship. Equal volumes of gases at the same temperature and pressure contain an equal number of particles. 1 mole gas = 22.4 L = 6.02 x particles at STP (273 K & 1 atm) molar mass volume 4 He222 Rn constant 22.4L
Therefore because of Avogadro’s Law if these three gases have the same number of particles and are at the same temperature and pressure, they must take up the same volume. HeRnO2O2
Molar Mass does not affect volume of a gas
D. Dalton’s Law of Partial Pressures: The sum of the partial pressures of all the components in a gas mixture is equal to the total pressure of the gas mixture. Ex #6) A flask contains a mixture of oxygen, argon, and carbon dioxide with partial pressures of 745 torr, atm, and 391 torr respectively. What is the total pressure in the flask? P T = P a + P b + P c + …
Dalton’s Law of Partial Pressures
Ex #7) The total pressure of a mixture of helium and neon is 498 mm Hg. If helium is 20.0 % of the mixture, what is the partial pressure of helium in mm Hg?
E. The Combined Gas Law: “Choyles” This law can be used to determine how changing two variables at a time affects a third variable.
Ex #8) A gas occupies 72.0 mL at 25 °C and 198 kPa. Convert these to standard conditions. What is the new volume? P 1 = 198 kPaP 2 = kPa V 1 = 72.0 mLV 2 = ? T 1 = 298 KT 2 = 273 K (273 K) ( kPa)
12.3 A. Ideal Gas Law Although no “ideal gas” exists, this law can be used to explain the behavior of real gases under ordinary conditions. P = pressure (atm) V = volume (L or dm 3 ) n = number of moles R = Latm/molK universal gas constant T = Kelvin temperature PV = nRT
B. Ideal Gas Law & The Kinetic – Molecular Theory: Under normal conditions (temperature and pressure) gases behave ideally.
n ____ P ____(more gas, ____________) more collisions
T ____ P ____ (moves faster, ______________)more collisions
V ____P ____(smaller volume, ____________) more collisions
2. Gases at low temperatures and high pressures do NOT behave ideally. As you decrease the volume of a gas, the volume of the particles themselves becomes significant. The Kinectic-Molecular Theory & Ideal Gas Law assumes the gas particles have no volume of their own. 3. Second, the attractive forces which are very small when the particles are moving fast, become larger as they slow down.
Ex #9) How many grams of carbon dioxide occupy a volume of 36.9 mL at 158 kPa and 72 o C? PV = nRT 1 st convert units to “R” units ( Latm/molK) (345 K) n = mol CO 2
C. Lifting Power of Gases: 1. Uses a gas “lighter” than air (smaller molar mass.) Ex) The Hindenburg used hydrogen Ex) Today’s blimps use helium 2. Hot Air balloons heat air to lower its density. 3. Effusion is the movement of gas atoms or molecules through a hole so tiny they move one particle at a time. Smaller particles effuse faster than larger particles. Ex) → → H 2 fastest He rare/expensive N 2 (air) plentiful/ cheap/slowest
The Hindenburg was a Dirigible (a balloon that has engines to control its horizontal movement)